Periodontal diseases are bacterial-associated inflammatory diseases of the supporting tissues of the teeth and range from the relatively mild form of gingivitis, the non-specific, reversible inflammation of gingival tissue to the more aggressive forms of periodontitis which are characterised by the destruction of the tooth's supporting structures. Periodontitis is associated with a subgingival infection of a consortium of specific Gram-negative bacteria that leads to the destruction of the periodontium and is a major public health problem. One bacterium that has attracted considerable interest is P. gingivalis as the recovery of this microorganism from adult periodontitis lesions can be up to 50% of the subgingival anaerobically cultivable flora, whereas P. gingivalis is rarely recovered, and then in low numbers, from healthy sites. A proportional increase in the level of P. gingivalis in subgingival plaque has been associated with an increased severity of periodontitis and eradication of the microorganism from the cultivable subgingival microbial population is accompanied by resolution of the disease. The progression of periodontitis lesions in non-human primates has been demonstrated with the subgingival implantation of P. gingivalis. These findings in both animals and humans suggest a major role for P. gingivalis in the development of adult periodontitis.
More recently there has been increasing linkage of periodontal disease and cardiovascular disease and therefore a link between P. gingivalis infection and cardiovascular disease. More information regarding this linkage can be found in Beck J D et al Ann Periodontol. 3:127–141, 1998 and Beck J, et al. J. Periodontol. 67: 1123–37, 1996.
P. gingivalis is a black-pigmented, anaerobic, asaccharolytic, proteolytic Gram-negative rod that obtains energy from the metabolism of specific amino acids. The microorganism has an absolute growth requirement for iron, preferentially in the form of haeme or its Fe(III) oxidation product haemin and when grown under conditions of excess haemin is highly virulent in experimental animals. A number of virulence factors have been implicated in the pathogenicity of P. gingivalis including the capsule, adhesins, cytotoxins and extracellular hydrolytic enzymes.
In order to develop an efficacious and safe vaccine to prevent, eliminate or reduce P. gingivalis colonisation it is necessary to identify and produce antigens that are involved in virulence that have utility as immunogens possibly through the generation of specific antibodies. Whilst it is possible to attempt to isolate antigens directly from cultures of P. gingivalis this is often difficult. For example as mentioned above, P. gingivalis is a strict anaerobe and can be difficult to isolate and grow. It is also known that, for a number of organisms, when cultured in vitro that many virulence genes are down regulated and the encoded proteins are no longer expressed. If conventional chemistry techniques were applied to purify vaccine candidates potentially important (protective) molecules may not be identified. With DNA sequencing, as the gene is present (but not transcribed) even when the organism is grown in vitro it can be identified, cloned and produced as a recombinant DNA protein. Similarly, a protective antigen or therapeutic target may be transiently expressed by the organism in vitro or produced in low levels making the identification of these molecules extremely difficult by conventional methods.
With serological identification of therapeutic targets one is limited to those responses which are detectable using standard methods such as Western Blotting or ELISA. The limitation here is both the level of response that is generated by the animal or human and determining whether this response is protective, damaging or irrelevant. No such limitation is present with a sequencing approach to the identification of potential therapeutic or prophylactic targets.
It is also well known that P. gingivalis produces a range of broadly active proteases (International Patent Application No PCT/AU96/00673, U.S. Pat. Nos. 5,475,097 and 5,523,390), which make the identification of intact proteins difficult because of their degradation by these proteases.